graft: drop cset description from empty commit message
This is either already redundant in the output or too verbose in quiet
mode.
graft: show hashes in user-facing messages
Graft was in various places just showing the revision number in status
messges. Instead, also show the stable and more useful short hash.
graft: give helpful warning for empty grafts
It was just showing a status message with the internal revision number.
Instead, show a warning like
note: graft of 27:
3aaa8b6725f0 "28" created no changes to commit
(message tweaked in-flight by mpm)